Radiation-sensitive elements having a laser-ablatable mask layer on the surface are known in the art. A relief image can be produced in such elements without the use of a digital negative image or other imaged element or masking device. Films with a laser-ablatable mask layer can be formed by first imagewise exposing the film with laser radiation (generally an infrared radiation laser under computer control) to selectively remove the mask layer in the exposed areas. The masking film is then placed in contact with a radiation-sensitive element and subjected to overall exposure with actinic radiation (for example, UV radiation) to cure the masking layer in the unmasked areas and to image the radiation-sensitive element through the mask. The film containing the mask layer and the imaged radiation-sensitive element (such as an imaged printing plate precursor) are then subjected to solvent development. The unexposed printing plate areas and the mask layer are completely developed off, and after drying, the resulting imaged element is useful, for example as a flexographic printing plate.
While flexographic printing plates having an integral laser-ablatable mask layer allow direct imagewise exposure using a laser and do not require a separate masking device, the time for imaging is generally too long since the system sensitivity to imaging radiation is low. Various attempts have been made in the industry to overcome this problem by increasing the infrared sensitivity of the mask layer. However, obtaining higher sensitivity has been a challenge due to the widely varying quality criteria that must be simultaneously satisfied. In addition, this approach requires the use of high-powered laser-equipped imaging apparatus that is especially configured for imaging flexographic articles. Because of the need for varying the thickness of flexographic plates depending upon the specific intended uses, more than one imaging apparatus may be required for the integral-mask approach.
An important advance in the art of making and using masking films is described in U.S. Patent Application Publication 2005/0227182 (Ali et al., hereinafter cited as U.S. Pat. No. '182). The described method provides a mask image in significantly less time due to greater imaging sensitivity.